The use of reciprocating pumps is well known in oil production wells. They generally consist of a polish rod which is longitudinally reciprocated within a pump casing mounted in series with an outer casing in the well. The polish rod cooperates with suitable check valves in the pump casing which serve to pump liquids to the surface of the well through the outer casing of the well. A drive unit is typically provided at the well head which serves to reciprocate the polish rod relative to the well casing.
Drive units commonly make use of linear hydraulic actuators coupled to the polish rod by various means. Typically, the cylinder end of the actuator is arranged to be fixed relative to the wellhead and the piston of the actuator is anchored relative to the polish rod such that extension and retraction of the linear hydraulic actuator results in a corresponding reciprocation of the polish rod relative to the well casing. Hydraulic actuators however are primarily designed with their linear extension force in mind, rather than lateral stability transverse to the direction of extension. When used for reciprocating a polish rod, the weight of the polish rod string can place considerable off-axis stresses on the actuators which might result in premature wear, or failure in some instances. Furthermore, due to the long stroke length of polish rod configurations, the connection of the polish rod to the actuators is necessarily at a high location above the wellhead which can be difficult or potentially unsafe to access for maintenance and the like.